This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Measurements using near infrared spectroscopy (NIRS) are abundant in muscle tissues, yet very little has been studied in terms of how optical properties changes in normal muscle tissues. We propose to study how various factors influence the optical properties of muscle tissues: (1) How do muscle optical properties vary with age, gender, and muscle fitness level? (2) How much do muscle optical properties vary on different timescales (daily, weekly)? (3) How do changes in muscle optical properties indicate response to exercise training (both short term and long term)?